I've been having this problem on one of my Joomla sites for a while....when trying to login at the backend, the page just refreshes to the login form again. The same thing happens for the login at the frontend.
After some searching, I came across this link but I've tried all the suggestions there and none work for me. Plus my database tables are all OK. On my local setup, the site works just fine...I just imported the database to the hosting server. Then used SSH to restore the zip file from my computer.
Any ideas?